Aerial Multispectral Agriculture Assessment Survey

​Normalized Difference Vegetation Index

To ensure the health of agricultural areas, regular inspections of the plant life is imperative. Because of their size and structure, they are often inspected from the air using multispectral aerial photography. Our pilots work with experienced engineers to produce detailed rendered inspection reports containing analytical information from aerial visual inspections. The colors we see in light are defined by the wavelength of that light. Plants absorb and reflect light differently depending on this wavelength. Plants typically absorb blue light and red light, while reflecting some green light. They also reflect a much larger amount of nearinfrared (NIR) light, which is not visible to the human eye but is visible to the camera system that we use. Reflectance is the percent of light that is reflected by the plant. By measuring the reflectance of a plant at different wavelengths, multispectral imaging enables identification of areas of stress in a crop, and provides a quantitative metric for the vigor of a plant.

Multispectral cameras work by imaging different wavelengths of light. The Federal Robotics multispectral camera has 5 imagers, each with a special optical filter that allows only a precise set of light wavelengths to be captured by that imager. Once processed, the output of the camera data is a set of images where the value of each pixel is equal to percent reflectance of light for that particular wavelength. These sets of images are then stitched together to create geographically accurate mosaics, with multiple layers for each wavelength. Mathematically combining these layers yields vegetation indices. There are many types of vegetation indices that measure different characteristics of a plant. Some indices, for example, are useful for measuring chlorophyll content of plant leaves. Other indices can be used to estimate nitrogen content.

Other indices provide indications of water stress. One popular index is the Normalized Differential Vegetation Index (NDVI), created by combining the reflectance from red and NIR light. Multispectral remote sensing provides radically new perspectives on the health and vigor of crops. It allows growers and agronomists to detect areas of stress in a crop and manage these issues immediately. It enables precise application of nutrient inputs and disease preventative actions based on the actual field conditions.

The widespread availability of low cost unmanned aircraft enables agricultural professionals to cost-effectively gather crop health information without waiting for satellite passes or paying the high costs of manned-aircraft flights. Imagery can be collected at resolutions measured in just inches per pixel. Data captured on a frequent basis enables growers and agronomists to map the health and vigor of crops today as well as observe changes in crop health over time.

Multispectral imaging enables collection of data in both visible and non - visible bands of light, allowing for generation of RGB color composite imagery as well as vegetation indices. Using high powered computer software, we are able to combine and render the multispectral data into insightful crop health maps.

NDVI (Normalized Difference Vegetation Index) reveals variability in plant vigor and biomass, often times not visible in standard RGB color imagery. Advanced vegetation indices like NDRE (Normalized Difference Red Edge) are more sensitive to changes in leaf chlorophyll content and provide information about plant nutrient status. The raw images can be transformed into geo-referenced multilayer orthomosaics, which can be downloaded and viewed using standard GIS applications. Each layer is registered at the subpixel level, with the value for each pixel indicative of percent reflectance for that band.